Distribution characteristics and correlation of macro- and microplastics under long-term plastic mulching in northwest China

Plastic film mulching applications can lead to high macroplastics (MaPs) and microplastics (MPs) concentrations in soils, adversely affecting soil health and crop production. However, the large-scale distribution pattern and correlation between MaPs and MPs presence in soils remain unknown. In this study, we selected 60 maize fields from 15 counties in northwest China to investigate the distribution and correlation of MaPs and MPs at different soil depths (0–10, 10–20, 20–30, 30–40 cm) and different plastic mulching years (5, 10, 15, 20 years). The results showed that MaPs are mainly concentrated at 0–10 cm soil depth, while MPs are distributed predominantly at 10–20 cm soil depth. Most MaP particles (45% of the total number) were within the 1–5 cm² size range, whereas MP particles in the 0.1–1 mm size range dominated (48%). An increase in MaP weight by 1.00 kg ha–1 increased MP weight by 0.27 kg ha–1, and an increase in MaP number by 1.00 × 104 pieces ha–1 increased MP weight by 0.90 kg ha–1. An increase in MP weight by 1.00 kg ha–1 increased MP number by 5.43 × 107 pieces ha–1. Fourier transform infrared spectroscopy and scanning electron microscopy results revealed degradation of the plastic surface, with further fragmentation into smaller particles likely. Based on our findings, we estimated that farmland in northwest China in 2020 contained 1.0 × 106 tons MaP and 2.7 × 105 tons MP. The severe MaP and MP pollution in China’s farmland requires improved recycling efficiencies and management practices of plastic film to ensure food security and sustainable agricultural development.